That line is a predicted track line for the next 60 seconds for that traffic. Other ADS-B out aircraft report their position every second, but it is common that the system might miss that report or even a couple. This is not that significant as they are every second, so you may only go a few seconds without a position update.
Now, where it really comes into play is for Mode C and Mode S aircraft being input into the ADS-B system. They are reported to the Radar system about every 10-15 seconds. So it they miss a report or so, it could be 30 seconds or more before you get an update. We project (based on the last known speed, track, vertical speed, etc) to estimate where that traffic is going to be in 60 seconds. As soon as we get a fresh update, we correct or update the actual position. This is why the system is not going to be great unit 2020 or until the majority of planes have ADS-B out.
Even come 2020, we all need to be sure to primarily keep our eyes out for traffic. The ADS-B system is only as good as the other aircrafts equipment, signals, antenna placement, and is affected more by Mode C/S traffic report delays. ADS-B Aircraft to Aircraft reports are very accurate. Just keep in mind that a Mode C/S aircraft is transmitting to the ATC Radar and the ground station
We really need to be careful and responsible for looking out for those Mode C/S aircraft. Not only until 2020, but even thereafter. Only those that are required to have a Mode C or S transponder, or plane to fly in to a Mode C area will have to upgrade to ADS-B compliant. That could potentially leave a fair number of airplanes that are not being updated with the ADS-B accuracy.
Dick, please don't take this as a lesson on ADS-B, I just want to make sure other readers can see the whole picture. I think there needs to be more clarification on ADS-B Traffic.